Almost have enough info now to help you.
In the group of grackles that have taken this liking to your trampoline... would you say their are more males then females in the group, more females to males, or pretty much even?
How old are the maple trees?
How tall are the fir trees?
And just guessing at it, how many branches do you think are on the oak trees?
When you say the trampoline is black do you mean a bright black or dull black? Also is the stand for it black also?
What color is your house?
When you park your car in the evening how close is it to the trampoline?
Do you have any neighbors within a 5 mile radius named Agnes?
What is your show size?